Evaluation of sound extinction and echo interference in densely aggregated zooplankton

Abstract The investigation of sound extinction and echo interference is important as regards the accurate assessment of the abundance of densely aggregated zooplankton. To study these effects, the analytical model describing sound backscattering by an aggregation of isotropic scatterer

Differences in swimbladder volume between Baltic and Norwegian spring spawning herring: possible consequences for mean target strength

Volume and shape of the swimbladder of Baltic and Norwegian spring spawning herring were measured, together with individual herring fat content. Swimbladder size was found to differ significantly between the two herring stocks. The larger swimbladder volume of Baltic herring is suggested to be associated with the fish’s low fat content, which may be linked to its specific energy budget and the low salinity of the Baltic Sea. A buoyancy model that considered the different salinity conditions and fat proportions was used to evaluate the observed differences in swimbladder volume. The swimbladder volume data were subsequently used to model the mean target strength (TS) as a function of depth and growth pattern. Backscattering of the swimbladder was modelled using the Modal-Based Deformed-Cylinder Model (MB-DCM), describing the swimbladder as a gas-filled elongated prolate spheroid. The fish body component was modelled as a fluid-filled ellipsoid using the Distorted Wave Born Approximation (DWBA). Modelling results support a different TS to size relationship for Baltic herring, with a stronger echo, due to the larger swimbladder. Depth and length dependent TS relationships based on the model results are suggested. Keywords: Baltic herring, swimbladder volume, depth dependence, target strength

On the echo interference in sound backscattering by densely aggregated targets

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in ICES Journal of Marine Science 62 (2005): 771-778, doi:10.1016/j.icesjms.2004.12.021.It is important to understand the volume backscattering by dense aggregations of a variety of scattering objects such as bubbles or biological targets. This paper addresses the interference of the echoes from randomly distributed targets. The main motivation of the paper is to understand the conditions under which the echo interference may affect the accuracy of the abundance and/or the biomass estimation in fisheries and zooplankton acoustics significantly. Our approach consists of two parts. The first includes an analytical approach, which describes explicitly the dependence of the echo interference on the pulse shape of the transmitted signals. Because of the limitations of the analytical approach, numerical computations based on Monte Carlo simulations of acoustic backscattering by three-dimensional target distribution were performed as a second approach. The impacts of the echo interference were studied numerically over a wide range of frequencies, for different pulse shapes and directivity patterns of the acoustic systems, and for various spatial distributions of the targets (abundance), as well as the corresponding target strengths. Using analytical and numerical approaches it was demonstrated that for targets that are uniformly distributed in space, the influence of echo interference on the observed volume-backscattering strength is strongly controlled by three main parameters. These are the number of targets in the sampling volume, the product of sound frequency and pulse duration, and the degree of tapering of the applied pulses. A numerical examples of the abundance estimation of marine organisms are presented.The work was sponsored by the Institute of Oceanology, Polish Academy of Sciences (sponsor programme 2.7), and by the USA's National Science Foundation (Grant # 010562900)

Comparisons among ten models of acoustic backscattering used in aquatic ecosystem research

Author Posting. © Acoustical Society of America, 2015. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 138 (2015); 3742, doi:10.1121/1.4937607.Analytical and numerical scatteringmodels with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m2) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not convergeThis work was supported by the NOAA Fisheries Advanced Sampling Technologies Working Group, the Office of Naval Research, and the National Oceanic Partnership Program. Josiah S. Renfree

Modelling herring target strength pressure dependence in the frequency domain

Improved understanding of the acoustic scattering properties of the fish is needed when trying to remove potential bias is acoustic abundance estimates. The paper address the problem of systematic pressure or depth dependency in herring target strength, as due to the uncompensated compression of the swimbladder with depth. As a first stage, a theoretical approach to the problem is developed using a Modal-Based Deformed Cylinder Model. This includes the pressure effects on the swimbladder and the effect of the fish flesh. Using the theoretical solutions obtained, a sensitivity analysis on fish target strength with pressure is made at various acoustic frequencies, and compared with real target strength data on adult herring

Mechanisms of sound seattering by biological targets and their aggregates

Natalia Gorska's thesis is based on a set of 9 papers published in scientific journals (Gorska &#x0026; Klusek 1998, Gorska 2000, Gorska &#x0026; Chu 2001a, b, Gorska &#x0026; Ona 2003a, b) and conference proceedings (Gorska &#x0026; Klusek 1994, Gorska 1999, Gorska &#x0026; Chu 2000), which broadly summarise her integrated research achievements in underwater acoustics from 1994 to 2003. She is the sole author of two of the articles (Gorska 1999, 2000), and is the first co-author, taking a leading part, in the others (Gorska &#x0026; Klusek 1994, 1998, Gorska &#x0026; Chu 2000, Gorska &#x0026; Chu 200la, b, Gorska &#x0026; Ona 2003a, b). <br>&nbsp;&nbsp;&nbsp;&nbsp;Her research objective was to work out the theoretical background to certain problems of sound scattering by biological targets - single individuals and aggregated layers of fish and zooplankton - in relation to environmental conditions in the sea. In the study she focused on acoustical extinction and backscattering, including the phenomenon of echo interference. In conjunction wit h the co-authors of papers Gorska &#x0026; Ona 2003a, b, Gorska &#x0026; Chu 2001a, b and Gorska &#x0026; Chu 2000, she was able to apply and verify her theoretical results empirically

Evaluation of sound extinction and echo interference in densely aggregated zooplankton

The investigation of sound extinction and echo interference is important as regards the accurate assessment of the abundance of densely aggregated zooplankton. To study these effects,the analytical model describing sound backscattering by an aggregation of isotropic scatterers (Rytov <i>et al.</i> 1978, Sun & Gimenez 1992) has been extended to the case of densely aggregated elongated zooplankton. The evaluation of the effects in the case of a dense krill aggregation demonstrates that they can be significant and should be taken into account

On acoustic multi-frequency species identification and separation of Atlantic mackerel, Norwegian spring spawn herring and Norway pout.

The accuracy of the multi-frequency species identification and separation algorithms may be significantly improved when the detailed acoustic scattering characteristics of the fish are known, including its expected variability and uncertainty. The modeling approach is effective for interpreting measured data collected for different fish species and may give a better understanding of the acoustic scattering characteristics. The paper is motivated by development of algorithms of multi-frequency species identification and separation of three economically valuable North European fish species: Atlantic mackerel, Norwegian spring spawn herring and Norway pout. The backscattering by these fish is studied using an advanced modeling approach, and the results of the analysis are compared with selected multi-frequency recordings. This allows for an explanation of the main backscattering mechanisms over the selected frequency range and often a better understanding of the dominating acoustic scattering parameters. The impact of fish orientation behavior, pressure (for herring) and the morphological state of the fish is also discussed. Typical backscattering frequency responses for herring, mackerel and Norway pout are explained theoretically. The obtained results may be a base for improving the automatic identification algorithms now introduced in scientific echo sounder post-processing systems